1. Field of the Invention
The present invention relates to systems to image gases using backscattered absorption gas measurements, and more specifically, it relates to backscatter absorption gas mapping systems for making measurements of gases in the environment.
2. Discussion of the Background
Most gases are invisible to the unaided human eye, particularly at low concentrations. It is thus difficult, and usually impossible, to visually determine the presence and extent of releases of these gases into the environment. The ability to rapidly detect and track hazardous gases in the atmosphere would greatly aid public safety and health, and would be useful in determining the source of gaseous leaks in general. For example, accidental toxic or combustible gas releases can occur from malfunctioning industrial equipment or from accidents involving the transport of bulk hazardous materials. These releases can rapidly diffuse into the surrounding air and move with the prevailing wind. While the safety of the public would be greatly enhanced in such circumstances by the easy determination of the location, extent, and motion of these gases, there is no device that is capable of providing this information. Existing detection technology is labor intensive and costly, requiring manually use of instruments that measure at a single point and in close proximity to the leak source.
Of particular interest is the determination of leaks of natural gas. The natural gas distribution system consists of networks of buried piping in which gas flows at relatively low pressure (15 psi). A leak generated in a distribution line emits natural gas into the surrounding soil where it migrates to the surface and can be detected. The nature of the transport is dependent upon the degree of compaction of the ground. In loose soil, gas is dispersed as it works its way upward to the air interface, where it is released as a diffuse plume. In soil that is more densely packed, the gas may seek low-resistance fissures through which it moves more easily than in the soil body. The boundary between the soil and the outer wall of the pipeline may also serve as such a conduction channel. Gas that has traveled through fissures may be more concentrated at the surface than gas traveling through loose soil—however, the fissure may guide it to a point that is distant from the location of the buried line.
Typically, the gas industry finds methane leaks using time consuming point measurement devices that are moved about in the vicinity of a suspected leak. One type of device has a sampling probe to provide air samples to a hydrogen flame ionization detector (FID). Since methane has a naturally occurring ambient concentration of about 1.5 ppm, the detector is set to produce an alarm for methane concentrations above the ambient concentration of about 1.5 ppm. Once the devices indicate the presence of methane, an operator moves a sampling probe or the entire device to find the source of the leak at the point of highest methane concentration. Depending on the sampling arrangement of the detector, leaks of 1-2 ppm can be detected.
Backscatter absorption gas imaging (BAGI) is one advanced technique that shows promise for remotely producing real-time images of methane and other gases. A BAGI system consists of a light source that produces radiation that is absorbed by a gas of interest and an imaging system that collects the light to produce images of the extent of the gas within an imaged scene. Light is directed towards an area having a solid object (e.g., a wall or the ground) in the imaging system's field of view. The solid object scatters light back towards the camera, and if the gas of interest is present, the light will be absorbed while traveling towards the object and when backscattered from the object to the imaging system. Light that is thus backscattered is imaged, or processed to produce an image, of the scene that can be interpreted by the BAGI system user to determine the presence and position of gas in the environment. A BAGI image, for example, can consist of light and dark regions according to the amount of absorbing gases present. Brighter regions correspond to scenes having no, or small amounts of, absorbing gases, and darker regions correspond to scenes having higher amounts of absorbing gases. By adjusting the wavelength of the BAGI light source to correspond to the spectral absorption features of different gases, BAGI systems can produce images of the extent of these different gases.
Although BAGI systems are able to provide an image of gas locations in the environment, prior art BAGI systems suffer from limitations that prevent them from being generally useful in producing real-time images of low concentrations of a gas naturally present in the environment. Problems in making BAGI measurements are generally related to the difficulties in obtaining backscattered light signals of sufficient strength and quality to differentiate localized gas absorption from the background. Thus, for example, some of the problems include, but are not limited to, variations in the optical properties of backscattering surfaces, scattering of light into collection optics, and detector sensitivity and noise. In addition, BAGI systems that illuminate a scene also have problems due to, for example, variations in laser power and difficulties in providing sufficient power at useful wavelengths of light.
In addition, since the BAGI image is derived from the total amount of absorbing gas along the system's optical path length, the presence of a background concentration is problematic. Thus, for example, at large BAGI system-to-backscattering surface spacing, the background absorption will dominate the absorption and the fugitive emissions will be comparable to the system resolution. Thus the maximum-light source-to-backscattering surface distance will be limited by the background concentration and resolution of the system. In addition, the distance from the light source to the backscattering surface can vary over the imaged scene. The absorption by the gas naturally present in the environment will attenuate the backscattered signal more for areas in the scene for which the backscattering surface is further from the BAGI instrument.
Another problem with prior art BAGI systems is due to “speckle” that results from reflecting a highly monochromatic light off of the uneven surface. Thus while the BAGI beams from the light source to the solid surface can have well defined and stable intensity profiles, the return beam has an speckle pattern resulting in intensity variations that introduce noise to the resulting absorption measurement.
Therefore, it would be desirable to have a method and system that provides a portable system to acquire images of gas in the environment, and thereby enabling the easy detection of leaks among a background of gas, that corrects for variations in the optical path length between different positions on a gas image, and that reduces some of the sources of noise in prior art systems. Such a method and system should be portable, robust, and capable of discriminating leaks of combustible gases from a safe distance.